With cold weather blasting in early this year, images of snowdrifts and downed power lines are in the news. If you are in charge of remote field operations, scenarios like this cause concern about the reliability of your assets, especially when access to the sites is blocked by the storm’s aftermath.
How reliable is solar power in sub-zero temperatures, snow, and ice? The answer is that if a system is sized precisely, with the appropriate components, to compensate for cold temperatures, shorter days, and periods of little or no sun, it will carry the load all winter without a hitch. That’s good news for difficult-to-reach locations because a downed system is expensive and can leave assets cut off from control centers.
When sizing a solar power system for cold regions, the winter sun hour rating for the location is absolutely critical. Never accept a system that has been sized based on the average annual sun hours because it will eventually fail during the long cold winter. The lower the sun hour rating for the location, the more solar panels and batteries are needed to collect and store energy when sunlight is weak and days are short.
The farther north the location, the steeper the solar array's angle to maximize sun insolation. In addition, the steep angle facilitates snow shedding off the face of the module to reduce snow accumulation. Also, check the average accumulated snowfall for the area. The solar arrays should be installed high enough to not be covered by snowdrifts.
As a battery discharges, the electrolyte solution converts to water, and water will freeze. If the batteries are over-discharged, not only will the system shut down, but the water in the batteries will freeze, expanding and splitting their cases. The swollen battery cases become wedged in the enclosure, impeding removal and replacement. To prevent this scenario, design the system with sufficient battery bank capacity to power the load through the period of reduced sun. As long as there is enough sun being captured to power the load and maintain the batteries to a high state of charge, the batteries will not freeze.
Batteries perform best at about 21C (70F). As temperature decreases, internal resistance increases which reduces the battery capacity. Temperature is a critical factor to consider; make sure to source reliable data for temperatures in the region before sizing the system. Here’s why: a battery that delivers 100% of its capacity at 70F may only deliver 50% at -18C (0F).
For example, according to East Penn’s specifications, its 110Ah rated battery delivers 100% capacity at +30C (86F). The same battery delivers only 50% battery capacity at -20C (-4F), or 55Ah. To compensate for the periods of extreme cold, the size of the battery bank must be doubled.
110 Ah Battery Energy Capacity at -20C = 50%, or 55Ah.
Based on the battery manufacturer’s specs, calculate the number of batteries required to power the load at the lowest predicted sustained temperature.
For applications at temperatures lower than -30C (-22F), Solarcraft recommends NiCad batteries. They tolerate extreme cold but are less efficient in charging.
A fully charged battery’s voltage varies with the battery’s temperature. As a battery temperature decreases, the battery charge voltage must increase. The charge regulator chosen for the application must be temperature compensated.
Battery float voltages may increase to as much as 30VDC as a result of the decreased temperatures and increased voltage. To avoid shutting 24VDC load components down due to increased voltage, the system may require the addition of a DC-DC converter between the battery and the load to correct the output voltage to 24.0 VDC.
Studies have indicated a slight increase in efficiency for every 1C decrease in ambient temperature—somewhere in the range of +0.05%. Although the increase is nominal, every bit helps when the days are short and the sunshine is weaker than in the summer months.
Choosing the best components for extreme temperatures is definitely worth the effort. Check the specifications for each of the load components inside the enclosure. Manufacturer temperature ratings should tell you a lot about which components to specify. You want to avoid using load components that will require heaters. Even a very small heater greatly increases the size of the solar array and battery bank in the winter.
If heaters are required for the load components, the heater will be the first load component to be disconnected due to a low voltage situation. The ambient cold temperatures will likely shut down the load components, and the system will go offline until the batteries can recharge to the correct float voltage. The best practice is to find components suitable for extremely cold locations and avoid the complications and costs associated with heating components.
To learn more about properly sized, hence reliable solar power systems for harsh winter conditions, contact a Solarcraft Technical Sales Representative at (877) 340-1224.